How to find which part of the source code is taking more time to execute in Java REST APIs built on SpringBoot?

I have a REST Web Service built on Spring Boot and consuming some back-end REST APIs and executing database operations along with some Redis cache transactions. It is taking more than 7 or 8 seconds to get response when i hit the API through Postman tool. Initially it was 2 or 3 seconds. When i see the executing time by using Java Date API before and after every operations like database transaction, redis cache transaction, consuming other REST services etc. and they are like 20ms, 90ms, 500ms etc., respectively. I am unable to find where the API response is getting delayed while executing the Java Code. (Also some asynchronous code is present in btw.)

For the time being, i cannot put the execution time difference across the source code due to huge amount of code and multiple service implementations in a single API call.

Is there an open-source tool to find out the source code execution time to understand the performance or response time of a REST web service to further debug or improve the performance easily ?

Any help is highly appreciated.